1. Field of the Invention
The present invention relates to a method for producing martensitic steel that comprises a content of other metals such that the steel can be hardened by intermetallic compound and carbide precipitation, with an Al content of between 0.4% and 3%.
2. Description of the Related Art
For certain applications, in particular for airplane engine transmission shafts, it is necessary to use steel having a very high mechanical strength (yield strength) up to 400° C., and at the same time good resistance to brittle fracture (high stiffness and ductility). These steels must also have good fatigue behavior.
One such martensitic steel is known from document U.S. Pat. No. 5,393,488, which includes a content of other metals such that it is capable of being hardened by an intermetallic compound and carbide precipitation. The composition of such a steel by weight is as follows: 10 to 18% of Ni, 8 to 16% of Co, 1 to 5% of Mo, 0.5 to 1.3% of Al, 1 to 3% of Cr, less than 0.3% of C, less than 0.1% of Ti, the rest being Fe.
The drawback of such a steel is its high cost, due to its significance Co content.
Also known as another martensitic steel that comprises the contents of other metals such that it is capable of being hardened by an intermetallic compound and carbide precipitation, the composition of which is given in document FR 2,885,142 as follows (percentages by weight): 0.18 to 0.3% of C, 5 to 7% of Co, 2 to 5% of Cr, 1 to 2% of Al, 1 to 4% of Mo+W/2, traces to 0.3% of V, traces to 0.1% of Nb, traces to 50 ppm of B, 10.5 to 15% of Ni with Ni≧7+3.5 Al, traces to 0.4% of Si, traces to 0.4% of Mn, traces to 500 ppm of Ca, traces to 500 ppm of Rare earths, traces to 500 ppm of Ti, traces to 50 ppm of O (development from molten metal) or to 200 ppm of O (development through powder metallurgy), traces to 100 ppm of N, traces to 50 ppm of S, traces to 1% of Cu, traces to 200 ppm of P, the rest being Fe.
This steel FR 2,885,142 has a very high mechanical strength (breaking load able to go from 2000 MPa to 2500 MPa) and at the same time very good resilience (greater than 180.103 J/m2) as well as a good compromise with the other properties of toughness and fatigue behavior.
However, the results of fatigue tests conducted on this type of steel by the inventors show great dispersion in the bench life values (corresponding to the number of cycles leading to the break of a fatigue test piece in said steel) for each imposed deformation stress level, whether for low-cycle fatigue (stress frequency in the vicinity of 1 Hz) or vibrational fatigue (greater than 50 Hz). Thus, the minimum values, within the statistical meaning, of the fatigue bench life (limiting the bench life of parts made from this steel) are still too low.